1. Field of the Invention
This invention relates to an optical system for observing an object image with the eye, used in finder systems of cameras for photography with silver halide, electronic photographing cameras, and in binoculars.
2. Description of the Related Art
Where organic materials are used in an optical system, it is generally known that water vapor is absorbed from the atmosphere into the optical system or moisture is discharged therefrom, thereby bringing about changes in refractive indices and dimensions of lenses to vary optical properties. As a provision for this problem, it is known, as disclosed by Japanese Patent Preliminary Publication No. Hei 3-181908, that optical elements constructed of organic materials are arranged inside lens barrels and the outermost lens elements coming in contact with the atmosphere are made from inorganic materials (although these materials are merely described as nonhygroscopic materials, they correspond to optical glass in view of the optical properties of the embodiments) so that inner humidity is held to the same state as in the case where the optical system is assembled with covers, and thereby the optical properties are constantly maintained without undergoing the influence of a humidity change of the atmosphere.
Further, as disclosed by Japanese Patent Preliminary Publication No. Hei 4-349418, it is known that thermoplastic resin with norbornane structure is used for at least one element of an objective lens to prevent the degradation of optical properties which are attributable to lens deformation due to hygroscopicity. Still further, as set forth in Japanese Patent Application No. Hei 5-113197 filed by the same applicant as in the present application, it is also known that the lenses of a focus detecting device are constructed of some material which is small in linear hygroscopic expansion coefficient (for example, polyolefin resin) to prevent the deterioration of focus detecting performance which is due to changes in refractive index and shape accompanied by the variation of humidity.
However, the above-mentioned prior art of Publication No. Hei 3-181908, which needs optical parts used as the covers, causes an increase in the number of parts. Moreover, since the encasement of movable lens units does not allow the operation of the movable lens units to be smoothly performed because of the effect such as that in an air pump, the optical parts serving as the covers, two for each movable unit, must be arranged. This causes a further increase in the number of parts constituting the optical system, and makes it practically impossible to apply the prior art to the optical system including a variable magnification section.
On the other hand, in the above-mentioned prior art of Publication No. Hei 4-349418 and Application No. Hei 5-113197, it is described that, on the assumption that the changes of refractive index and shape caused by hygroscopicity are homogeneously produced, low hygroscopic materials are used for lens elements which are easily influenced by the changes and have powers from the first. A change in homogeneity, however, occurs in the saturated state of hygroscopicity, and for practical use, the refractive index existing close to the surface of a lens will differ from that at the center thereof so that what is called inhomogeneous distribution is produced. In this case, even with optical elements having no power from the first, the power by index distribution will be generated. When hygroscopicity is saturated and homogenized, the optical elements will return to the state where they have little power.
However, none of the prior art recognizes the presence of the above problems, not to speak of the technique of diminishing such transient variations of characteristics.